I wrote this program before I started using sysexits. Now, a different
exit code makes more sense.

* -V and --version now print the program version and exit
* Use the `exitcode` crate
* Rename `options` variable to `accepted_args`, which seems clearer
  given its usage.

When investigating how to add a `--version` argument, I was confused by
this naming and thought that "options" referred to command line options.

Rename the variable to make its purpose clearer.

* Remove old in-progress code
* Remove `child.stdout.take()` call, which is suggested in
  https://doc.rust-lang.org/std/process/struct.Child.html#structfield.stdout
  but ended up not being necessary in this instance.

Previously, Passextract didn't work with GnuPG 2 because I was
immediately collecting the output of `pass`.

With GnuPG 1.4, the password prompt was like a conventional `read`-style
password prompt. However, in GnuPG 2, the prompt is replaced with
Pinentry. Pinentry interferes with capturing the output from Pass, such
that Passextract exits immediately, without even prompting for a
password.

Instead of using `Command.output`, use `Command.spawn`. This allows
GnuPG to work correctly and display a Pinentry prompt. We can then
capture the standard output from the child process and parse it for the
Passextract interface.